The default value is UIRectEdge.All.
Prior to iOS 7, the UIViewController.View did not visually underlap parent elements such as the status bar, navigation bar, or toolbar. In iOS 7, it typically should.
The UIViewController.EdgesForExtendedLayout specifies to the UIViewController.ParentViewController how the edges of this UIKit.UIViewController should be extended for underlap. The default value of UIRectEdge.All specifies that all edges should be extended to underlap, while UIRectEdge.None specifies an extent similar to that in iOS 6 or earlier.
The following image illustrates the difference: when UIKit.EdgesForExtendedLayout is set to UIKit.UIKit.UIRectEdge.All, the drawing rectangle of the underlying UIKit.UIView underlaps, but if UIKit.EdgesForExtendedLayout is set to UIKit.UIKit.UIRectEdge.None, the drawing rectangle is restricted to the interior.